$ { {1} \times \left[\begin{array}{rrr}{1} & {-2} & {1} \\ {2} & {0} & {-1}\end{array}\right] = {?} } $
Solution: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rrr}{1}\times{1} & {1}\times{-2} & {1}\times{1} \\ {1}\times{2} & {1}\times{0} & {1}\times{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rrr}{1} & {-2} & {1} \\ {2} & {0} & {-1}\end{array}\right]}$